Stage I, II, etc. ???

I know I'm probably being extremely lazy not figuring this out for myself but there's some really smart, techie types on here who I'm sure can answer quickly and clearly.

What exactly is involved in the various upgrades referred to as Stage I, Stage II, etc? Are there different options within each one or do you have to do all of them to be considered a complete upgrade? Any other benefits besides performance and resale?